Preheat broiler. Place tomatoes and jalapeño cut-side down in a baking dish. Cut
one of the onion halves in half and add to the baking dish. Roast the vegetables
under the broiler 8 to 10 minutes or until they begin to blacken. Remove from
the oven and cool. Transfer the vegetables and accompanying liquid to a blender
and pulse until thick and chunky.

Dice remaining three-quarters of the onion for a large skillet, heat the olive
oil over medium-high heat. Add onion and garlic and sauté for 5 minutes. Add
spice mix and sauté for 3 to 4 more minutes. Add chicken pieces and sauté until
the chicken is heated through, stirring often. Add tomato mixture to the skillet
and deglaze, stirring and scraping the bottom of the skillet to loosen any
browned bits. Add cilantro. Bring mixture to a simmer. Simmer 10 minutes. Remove
from heat and use as a filling in enchiladas or add beans and use as a filling
for burritos. Top with cheese, shredded lettuce and guacamole, sour cream or
salsa.